"seems like best" isn't always best.
People are dying therefor your other healers obviously need help with the reactive healing.
example: You spend 6 global cooldowns shielding 6 people who take 3k damage.
Those 6 global cooldowns could have been spent topping off people with your great borrowed time hasted casts, while a holy priest could spend 1 global cooldown on a circle of healing hitting those 6. Or even 1 bubble for borrowed time and a prayer of healing on a target damaged group.
Just because the end result is you "healed more" with your prevention doesn't mean it's always the best use of your time, what else could you have been doing in those 6 global cooldowns of "guessing" where damage is going to come.
Those 6 people you shielded could be fine while someone else dies, those 6 people might never be in danger of death even without shields, while 2 other people you didn't shield or had shielded earlier might die while you're shielding where a reactive penance or flash heal might have easily saved them.
I'll say it again there's certain fights with mechanics of predicable burst when it may be a good thing mass shielding, for the most part it is not and creates weakness in yours and your other healers abilities to deal with incoming damage.
As for holy "resting", it's rather rare, mending and renew are extra powerful global cooldown spells as holy, extremely high % of the time there's a surge of light proc after a CoH or PoH cast, as well as normal flash heal fillers for serendipity when the next PoH is needed. The only real time i deliberately "rest" as holy is when fight mechanics allow it or deliberately out of 5sec with inner focus and devine hymn.
Holy Concentration is not what it once was, out of combat regen stopcasting doesn't come nearly so easily.